1. Field of the Invention
The present invention relates to an air-fuel ratio control device for an internal combustion engine for an outboard motor, an air-fuel ratio control method, and a program product. The present invention is particularly preferred when used for controlling an air-fuel ratio of the internal combustion engine for an outboard motor to be a predetermined air-fuel ratio on a lean side.
2. Description of the Related Art
Conventionally, when it is attempted to control an air-fuel ratio of an internal combustion engine, an air-fuel ratio sensor and an O2 sensor disposed in an exhaust system of the internal combustion engine are used. The air-fuel ratio sensor is able to detect the air-fuel ratio accurately in a wider range than the O2 sensor, but is more expensive than the O2 sensor and causes increase in cost of the internal combustion engine. On the other hand, the O2 sensor is less expensive than the air-fuel ratio sensor but is able to detect the air-fuel ratio only in the vicinity of a logical air-fuel ratio. Specifically, the O2 sensor is only able to detect whether the actual air-fuel ratio of the internal combustion engine is on a lean side or a rich side from the logical air-fuel ratio.
On the other hand, in order to improve fuel consumption, the engine is operated with the air-fuel ratio being changed to a predetermined air-fuel ratio on a lean side from the logical air-fuel ratio in some cases. In such cases, when the actual air-fuel ratio is the predetermined air-fuel ratio on the lean side, it is possible to improve the fuel consumption, but due to dispersion of parts such as injectors for example, the actual air-fuel ratio may be displaced from the predetermined air-fuel ratio on the lean side. However, the O2 sensor only detects whether the actual air-fuel ratio is on the lean side or the rich side from the logical air-fuel ratio as described above, and it is not able to detect whether or not the actual air-fuel ratio is at the predetermined air-fuel ratio on the lean side.
Regarding such problems, in Patent Document 1, the logical air-fuel ratio is taken as a target air-fuel ratio for operation, and the displacement from the actual air-fuel ratio is corrected using the O2 sensor while calculating a feedback correction coefficient by feedback control. Next, a learning correction coefficient is calculated from the feedback correction coefficient, and open loop control is performed by applying the calculated learning correction coefficient, so as to control the actual air-fuel ratio to be a predetermined air-fuel ratio on a lean side. Therefore, by the air-fuel ratio control for the internal combustion engine described in Patent Document 1, it is possible to control the actual air-fuel ratio of the internal combustion engine to be the predetermined air-fuel ratio on the lean side even by using the O2 sensor, thereby achieving improvement in fuel consumption.    Patent Document 1: Japanese Laid-open Patent Publication No. 57-105530
The outboard motor can be mounted on various types of hulls, which is different from vehicles such as motorcycles and automobiles. For example, the outboard motor can be mounted on a high-speed vessel or heavy vessel, or plural outboard motors are mounted on one hull in some cases. Thus, when the use environment is different, there occurs a displacement of the actual air-fuel ratio from the target air-fuel ratio in the internal combustion engine.
Further, alcohol-mixed gasoline as fuel for internal combustion engines is increasingly used particularly in other countries. The logical air-fuel ratio differs between genuine gasoline and alcohol-mixed gasoline, and thus a fuel injection amount and so on for the internal combustion engine differ as well. Therefore, also when the fuel is changed from the genuine gasoline to the alcohol-mixed gasoline, the displacement of the actual air-fuel ratio from the target air-fuel ratio occurs in the internal combustion engine.
When the operation continues while the actual air-fuel ratio is displaced from the target air-fuel ratio as described above, it is possible that the improvement in fuel consumption is not achieved or that it causes an unpleasant sensation in operational feeling of the boat operator. Therefore, it is desired that the learning correction coefficient is calculated early and the calculated learning correction coefficient is applied, so that the actual air-fuel ratio matches the target air-fuel ratio in a short time. On the other hand, when importance is placed only on calculation of the learning correction coefficient early and an inaccurate learning correction coefficient is applied, the original object to match the actual air-fuel ratio with the target air-fuel ratio is impaired.